wow, no experience with that so no idea on the fungal infection.
if about to toss out, maybe lift plant from pot a bit and inspect roots? i would think it would be a clear sign in root zone if it is an infection?
you should clear out dead, fallen leaves. this is an opportunity for microbial growth - good or bad. I see dead leaves in pots last 4 weeks in pictures. probably not the cause here, i'd assume.
those tall ones look like they started to go downhill after week 7 or 8. Lost their perkineess.
anything newly added since week 7? it may have taken a while to build up severity... even anything new (product or behaviour) a week or two before that.